Some Last-Minute Tips on Soybean Diseases

As I was digging weeds already out of the garden this weekend and looking at the buds on the trees, it looks more like early May rather than early April.

Either way we are getting our seed together for research plots and checking our list of locations to begin for what is already an unusual season.

While the rain is keeping us at bay - here are some reminders of last minute checks and to do's before it turns into those crazy long days.

Sample for SCN.

We've always recommended sampling in the fall after a soybean crop.

This is primarily due to when SCN populations will be the highest as well as more time in the fall to sample these fields.

Fall sampling has not been ideal the last few years.

This year it is very unlikely that there was any winter kill of eggs, those that are not protected by the cyst wall.

Based on Laura Lindsey and Terry Niblack's recent survey most of our fields have some SCN and they are below the economic threshold.

However, there were also some surprises and we know historically that once populations get super high (>15,000 eggs/cup of soil or 60 cysts) that it is very difficult to get those populations to drop with just crop rotation.

Not to mention the consistent loss in yield over time.

The top fields to target this spring would be:

Fields where the yield is consistently 10 to 20 bu off your farm or county average.

Fields that have been in continuous soybean (>5 years)

Fields that have a proliferation of winter annuals or cover crops that are also great hosts for SCN (legumes, purple dead nettle, & Sheperds purse)

For a refresher on how to sample for SCN, here is a link to sampling by one of our newly minted Ph.D. students (#8 in the series)

Most farmers in Ohio know which fields have had diseases, Phytophthora root and stem rot, Sclerotinia stem rot, Frog eye leaf spot and replanting due to seedling diseases.

All of these are best managed with resistance.

Each company uses a different scoring system.

1 to 9 - where 9 is dead and 1 is little to no disease

1 to 9 - where 9 is best - top rating and 1 is tons of disease and plants are dead

As you look at these ratings you are going to see that most of the varieties are in the range of 3 to 7 - in the middle.

This is because the resistance that we most commonly deploy takes many genes (quantitative) to manage the particular disease and under conditions that are highly favorable for disease development some disease will occur.

A good example is Sclerotinia stem rot - varieties with the highest resistant ratings will develop 6 to 15% incidence under high disease conditions, well below where overall yield loss can occur.

Do you have a seed treatment?

Is it the right package for your conditions?

Replanting is costly.

From a seed, herbicide and delayed planting perspective.

In Ohio, soybean seedling pathogens are very diverse and include the watermolds (Phytophthora and Pythium) as well as true fungi (Fusarium spp. and Rhizoctonia).

All of these are very well managed with seed treatments, but there is not one fungicide that will control this plethora of pathogens.

The mainstay, metalaxyl/mefenoxam is very good for P. sojae and some Pythium spp, the same is true for ethaboxam.

A strobilurin will have efficacy towards some Pythium spp. and some true fungi.

Fludioxonil and several other chemistries will manage Rhizoctonia and Fusarium,.

The key for today's seed treatment mix is to be sure you have several fungicides in the package,that will actually mix and work well together.